Dissertation Proposal - The University of Arizona Campus Repository

I - INTRODUCTION
The fitness of any evolutionary unit can be understood in terms of its two basic
components: fecundity and viability. As embodied in current theory, the trade-offs
between these fitness components drive the evolution of life-history traits (Stearns 1992).
In unicellular individuals, the same cell must be involved in both fitness components,
typically these components being separated in time. However, in multicellular organisms,
under certain circumstances, cells may specialize in one component or the other, the
result being a division of labor, leading to the differentiation of germ and soma. The
evolution of a specialized and sterile soma can increase viability and indirectly benefit
fecundity (e.g., increasing nutrient uptake) but, all things being equal, must directly cost
fecundity by reducing the number of cells producing offspring. On the other hand, the
evolution of a specialized germ will benefit fecundity (by reducing the generation time
and/or increasing the quality of offspring), but must directly cost viability by reducing the
number of cells participating in viability-related functions.
Various selective pressures may push unicellular organisms to increase in size, but
general constraints, such as the decrease in the surface to volume ratio, set an upper limit
on cell size. Increase in size can also be achieved by the aggregation of mitotic products
that are held together by a cohesive extra-cellular material, increasing the number of cells
(instead of cell size). Natural selection has favored this strategy as illustrated by the
multiple independent origins of colonial and multicellular organisms in, for example,
algae (Niklas 1994; 2000). Large size can be beneficial both for viability (e.g. in terms of
predation avoidance, ability to catch bigger prey, a buffered environment within a group)
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